The present invention relates to a delay line for a cylindrical travelling-wave tube, particularly an amplifier, as well as to the tube equipped with such a line. It relates more particularly to the realisation of the variable pitch in such a line. The delay line according to the invention is considered to be of the type with tops and rings.
It comprises a metal guide, e.g. a cylindrical guide and, in the cross-sections of said guide, coaxial metal rings, each of which is fixed to the guide wall by at least one metal rod. It also comprises at least one top or cover constituted by a metal wedge fixed to the inner wall of the guide and extending over the entire length of the latter.
In the tube, the electron beam is transmitted along the preceding guide axis which, without it being obligatory, may constitute the vacuum envelope of the tube. The operation of the tube is based on the interaction between the electromagnetic waves which are propagated along the delay line and the electron beam.
Such lines are known in the travelling-wave tube art, particularly from U.S. Pat. No. 2,942,143. They are formed by a succession of elementary portions or cells, which are all identical and occur periodically along the transmission axis of the electron beam. There are a number of variants thereof, whereof one provides for the electrical interconnection of the tops by a single metal wire, more particularly forming a closed loop in order to eliminate certain interfering operating modes and when the line has a plurality of tops, cf U.S. Pat. No. 3,353,121. In this way, even if said interfering modes are not eliminated, they are at least transferred to frequencies outside the operating band of the tube on the principle mode. Thus, for example, in the case of a tube operating in the S band at about 3 gigahertz, the interfering modes in question are generally transferred to about 6 gigahertz.
The interest of a variable pitch for said lines was soon recognised in the travelling-wave tube art, more particularly for the reason of the efficiency of the tubes in which they are incorporated. The pitch of a delay line constituted, like those involved here, by a regular alignment of identical elements, is the distance between homologous points of two consecutive elements.